Direct and indirect mortality from temperature and disease
Heat mortality is direct: high temperatures and humidity exceed the body's cooling capacity, causing heat stroke. A wet-bulb temperature (the lowest temperature achievable by evaporative cooling) above 35 degrees Celsius is fatal even at rest; above 31 degrees, prolonged exposure becomes dangerous. Death rates in heatwaves scale exponentially with peak temperature: a 35-degree heatwave in a temperate region (unaccustomed population) causes 10-50 excess deaths per million people; in tropics it may be lower (populations adapted), but as heat intensity increases, even adapted populations suffer. The 2021 Pacific Northwest heatwave killed over 1,400 in the region; the 2010 Russian heatwave killed 55,000. At 2 degrees warming, heatwaves will be routine; at 3 degrees, heat-stress deaths could exceed 100,000 per year globally.
Indirect mortality follows disease vectors and nutrition. Warmer temperatures expand the geographic range of malaria, dengue, and tick-borne illnesses. Disrupted harvests from drought and flooding reduce food security, increasing malnutrition and child mortality. Air quality worsens as wildfires increase and ozone production rises with temperature, exacerbating respiratory disease.
Healthcare system strain and equity disparities
Climate impacts are not evenly distributed. Poor and marginalized communities have less access to air conditioning, less ability to relocate from hazardous zones, and weaker health infrastructure. Heat mortality is 1.5-3 times higher in low-income neighborhoods of the same city. Tropical and subtropical developing nations (with the highest vulnerability) have the fewest resources for adaptation. This amplifies existing health inequities: malaria, diarrheal disease, and malnutrition already kill millions in developing regions; climate change makes them worse.
Healthcare systems in vulnerable regions are already strained. Severe heatwaves, flooding, and disease outbreaks occurring simultaneously (compounding extremes) exceed hospital surge capacity. Mental health impacts (anxiety, depression, PTSD from extreme events) are increasing, especially among youth aware of long-term climate risks. The WHO estimates climate change will cause 250,000 additional deaths per year by 2050 from malnutrition, malaria, diarrhea, and heat stress combined, dwarfing current infectious disease mortality in many regions.